A metal having a smaller particle diameter is known to exhibit physical properties particular to the size. In particular, nano-scale particles may exhibit properties different from those of a bulk material. A material for bonding different substances using metal nanoparticles, in which such properties are used, has been proposed.
On the other hand, when the electronic control of an automobile and an industrial machine is promoted, power consumption increases. Therefore, the operating temperature of a semiconductor used in the inside thereof tends to increase. For this reason, a bonding material that can withstand a high-temperature environment is desired. Conventionally, lead-containing solder that can maintain its strength at high temperature has been used. However, from a recent tendency of reducing the use of lead, the use thereof tends to be restricted.
A temperature at which a bonding portion is exposed during operation may be near the melting point of solder. In this case, a bonding material having a higher melting point is required. However, when a brazing material having a high melting point is used, a bonding method is not facilitated. Therefore, the provision of a bonding material that is capable of bonding at low temperature and has bonding strength even at high temperature, and a bonding method has been desired.
In order to apply a technology that requires the application of pressure to materials to be bonded during bonding, a device capable of applying at least pressure and heat simultaneously is required. This technology somewhat has a problem in versatility. The technology has another problem in which it cannot be used for a material having only a mechanical strength enough not to withstand the application of pressure. Therefore, if a paste (bonding material) that can exhibit appropriate bonding power without application of pressure to the materials to be bonded can be provided, an object to which the technology can be applied is expected to significantly increase.
An atmosphere where a bonded body is formed is an oxidative atmosphere containing oxygen, for example, at least in the air. For this reason, in a bonding portion containing a surface to be bonded and a bonding material, the bonding portion may be oxidized to adversely affect the bonding power. An influence of oxygen on the bonding portion may be remarkable particularly on a fine bonded body. In order to eliminate such an influence, bonding may be performed under an inert atmosphere including nitrogen. If a bonding material that can exhibit sufficient bonding power even under an inert gas atmosphere can be provided, the use field and potential of the bonding material are expected to significantly increase. Therefore, bonding without application of pressure is strongly required.
As a candidate of a bonding method that can meet such requirements, a bonding method using silver nanoparticles, in which bonding can be achieved using no lead at a temperature lower than that of silver of a bulk material, recently receives attention. For example, Patent Literature 1 discloses that a three-metallic-component type composite nanometallic paste that contains three types of metallic component particles of large, medium, and small sizes in terms of specific size, and particularly composite metal nanoparticles as particles of small size, is sintered in an inert gas without application of pressure, to form a compact metal bonding layer having a high bonding strength of 10 MPa or more.